We have made direct measurements of the properties of the channels through which calcium enters parathyroid cells under normal physiological conditions. These channels are voltage-independent, and their single-channel conductance at the physiological calcium concentration is about 0.6 pS. We are developing an improved assay for parathyroid hormone (PTH) secretion in order to determine how the secretion of PTH is affected by different divalent cations, and how it is affected by gradients of osmotic pressure across different components of the parathyroid cell. We are using two complementary techniques to determine the properties of ionic channels in the membranes of secretory vesicles of the neurohypophysis. One technique involves measurement of the channels after they have been incorporated into lipid bilayers, and the other technique involves immunopurification of mRNA for the channels from Polysomes and expression of this mRNA in frog oocytes.